Loud speaker system

ABSTRACT

This invention provides a loudspeaker system having two coaxial cone diaphragm type nested speakers (woofer and tweeter) for handling an extended range of audio frequencies. The speaker system embodying the invention is characterized by a phasing baffle plate extending part way from a region spaced near the rim of the tweeter toward the woofer cone diaphragm. By controlling the dimensions, spacing and material of the phasing baffle plate, the overall frequency response for the two speakers may be rendered substantially uniform over the extended range.

United StatesiPatent [1 1 Torn [ LOUD SPEAKER SYSTEM [75] lnventor:William R. Torn, St. Charles, 111.

[73] Assignee: Dukane Corporation, St. Charles, 111.

[22] Filed: Aug. 30, 1972 [21] Appl. No.: 285,012

[52] US. Cl. 179/116, 179/115.5 R

[51] Int. Cl H041 9/06 [58] Field of Search l79/115.5 R, 115.5 PS,'179/116; 181/31 R, 31B

[56] References Cited UNITED STATES PATENTS 2,757,751 8/1956 Tavares181/31 B 2,496,589 2/1950 Marquis l79/115.5 PS

2,047,290 7/1936 Ringel t r 181/31 B Engholm 179/116 1 Mar. 12, 1974Primary Examinerl(athleen' H. Claffy Assistant Examiner-Thomas L.Kundert Attorney, Agent, or Firm--Robert L. Kahn 57 ABSTRACT Thisinvention provides a loudspeaker system having two coaxial conediaphragm type nested speakers (woofer and tweeter) for handling anextended range of audio frequencies. The speaker system embodying theinvention is characterized by a phasing baffle plate extending part wayfrom a region spaced near the rim of the tweeter toward the woofer conediaphragm. By controlling the dimensions, spacing and material of thephasing baffle plate, the overall frequency response for the twospeakers may be rendered substantially uniform over the extended range.

4 Claims, 10 Drawing Figures LOUD SPEAKER SYSTEM This invention relatesto a loudspeaker system of tht type having nested coaxial cone diaphragmtype speakers for extended frequency response. The larger speaker(woofer) is relied upon for handling the lower frequency range, whichmay extend from the lowest frequency to be handled, generally somethingless than about 50 Hz., to somewhere in the region of about 1,200 Hz.Nesting within and coaxial with the conical region defined by the wooferdiaphragm is a smaller speaker (tweeter) which is suitably supported onthe woofer. The tweeter is relied upon to handle frequencies from theorder of about 800 Hz. to the maximum value to be handled, such maximumvalue being about 8,000 Hz. to 16,000 or more, depending upon the topfrequency to be handled by th speaker system.

The tweeter cone diaphragm may have a diameter in the range of about 6inches to about 3 inches and the woofer cone may have a diameter in therange of about 18 inches down to about 8 inches. The tweeter has its potor permanent magnet field structure usually carried by the permanentmagnet structure of the woofer so that the voice coils and diaphragms ofthe two speakers may vibrate independently. Voice currents over theentire frequency range to be handled are fed to both speakers in whichcase the mechanical properties of the woofer and tweeter may be reliedupon for separating the low and higher frequency ranges. Or electricalnetworks may be provided for frequency separation of voice currents tothe woofer and tweeter, there being a cross-over range of frequenciesfrom about 800 Hz. to about 3,000 Hz.

A speaker system embodying the present invention is used as conventionalwoofer and tweeter speaker combinations, with conventional baffle platefor the woofer, all housed in a conventional cabinet. A cabinet forhousing a tweeter andwoofer combination, whether embodying the presentinvention or not, may have an open back or may utilize a substantiallysealed cabinet having acoustic absorbing material within the cabinet forloading. In such a speaker system, it must be borne in mind that thefrequency cross-over network, if provided, or the mechanical response ofthe woofer and tweeter is such that from about 800 Hz. and higher, theresponse of the woofer becomes weaker with frequency rise while thetweeter response increases. This gradual shifting of the speakerresponse is thus generally operative over a substantial frequency rangefrom the order of about 800 Hz. to the order of. about 3,000 or more Hz.

PRIOR ART AND ITS DRAWBACKS In designing a high quality audio frequencyamplifier and speaker system, the frequency response characteristics ofan amplifier as one means and that of a speaker system as a transducerare separately designed. This is due to the general practice ofengineering and production of audio frequency amplifiers as one field ofcommercial activity and speakers or speaker systems as another field ofcommercial activity. Even if one source provides amplifiers andspeakers, the above practice of separately engineering the two is stillgenerally true. The technical qualifications for the two fields arequite different.

In theory, an audio frequency amplifier having a uniform frequencycharacteristic when feeding a high quality speaker system also having auniform frequency response characteristic should provide an overalluniform frequency response over the frequency range being handled. Inpractice, this ideal is rarely if ever realized.

As a practical matter, the frequency response characteristic of aspeaker or speaker system usually repre sents an idealizedcharacteristic derived from smoothing out the actual frequency responsecurve. This may be accomplished by rapid frequency changes over thespectrum. Generally, it is not possible to produce a speaker system andits component parts free from some sharp resonance peaks oranti-resonance holes (dips) in the desired working range. The difficutlyis due primarily to the multiplicity of separate physical parts in aspeaker system, each having individual resonances an being physically oracoustically coupled to each other. Consequently over the entirefrequency spectrum to be handled by a speaker system, it becomes wellnigh impossible to avoid some resonant peaks or anti-resonant holeswithin the normal listening range. The factors which may be involved inhaving undesired peaks and holes in the speaker response may relate tosuch physical elements as the speaker baskets, the cone diaphragm andsuspension for either or both speakers, the nature of parts andairchambers providing physical or acoustic coupling between the twospeakers, the voice coils and mountings.

When a speaker system exhibits resonance peaks or holes of substantialamplitude, a painstaking analysis of possible sources of trouble as wellas some physical changes in the speaker system may be required toimprove the frequency response characteristics. Such a program forcritical analysis and possible physical changes requires not only asubstantial amount of time and expenses for engineering but may inaddition require changes in tooling for the manufacture of suchspeakers. The testing requirements involve actual production sampleswith production tolerances. Thus unlike most production procedures,where engineering or production is condemned because of costs in toolingor production changes, speakers require production units for testing.

The depth of the tweeter ,within the woofer cone may be varied andeffect some changes (good or bad) in the frequency responsecharacteristics of this speaker combination. For the most part however,such a procedure does not alter the basic frequency characteristics. Thegenerally rough frequency response characteristic of such a speakercombination usually remains.

THE INVENTION GENERALLY AND ITS ADVANTAGES The invention generallycontemplates the addition of an annular member of suitable materialattached to one of the speakers, preferably the tweeter, at theanchorage of the large tweeter cone diaphragm end and within the spaceencompassed by the woofer cone diaphragm. The annular member ispreferably flat, extending beyond the rim of the tweeter diaphragm in aplane generally normal to the coincident cone axes. The nature of thematerial may vary widely and should be self supporting or havereenforcements to provide a selfsupporting structure and such materialshould have good acoustic properties. Such materials as natural orsynthetic woods, wall-board, plastic sheet, particle board material maybe used. Any material that is useful as a sounding board may be used.The thickness is one of the dimensional parameters which must bedetermined by testing. Generally, material from about oneeighth inch tothree-fourths inches may be used. While metal may be used, it should bemuch heavier than conventional sheet metal. As a rule, metal should beabout one-sixteenth inch or thicker. Lighter materials like Styrofoam,having a smooth outer molded skin, are satisfactory in which case athickness of about threeeighths inch to about three-fourths inch may begood, depending upon desired frequency characteristics.

Considerable latitude in the shape of inner and outer plate edges, platethickness, spacing between inner plate edge and tweeter cone outer edge,spacing between plate outer edge and woofer cone wall is permissible,all such variables affecting the frequency response of the speakersystem. The space between the outer edge of the phasing baffle plate andadjacent woofer cone wall should be at least about an inch for a woofer,12 inch cone and more for a 15 inch cone. Too narrow a space between thewoofer cone wall and plate outer edge tends to throttle low frequencyresponse.

The invention directs engineering efforts to improve speaker responsefrom a production speaker system (that has been reasonably welldesigned) to a phasing baffle plate by controlling the above identifiedparameters. The relative spacing between woofer and tweeter within thewoofer cone and relative spacing of phasing baffle plate along the coneaxis are also parameters which can be operated on with engineering andtesting. Once a phasing baffle plate for a particular speakercombination has been satisfactorily determined, then quantity productionof the entire speaker system can proceed. The selection of relativewoofer and tweeter sizes will be governed for the most part byconventional engineering technique. There are so many variables inadapting a phasing baffle plate to a speaker combination that animproved frequency response characteristic can be obtained byconcentrating engineering changes on the phasing baffle plate and, forthe most part, leaving the woofer and tweeter speakers as is.

Commercially, the invention makes possible the quantity production ofspeaker combinations having exceptionally desirable frequency responsecharacteristics at a cost substantially less than conventional highquality speaker systems.

In a simple embodiment of the invention, a phasing baffle plate is flatwith a suitably shaped window therethrough providing an inner edge whichwill normally be partly or wholly spaced from the mounting rim of thetweeter to provide an air path. The extent of the air path is animportant parameter and is experimentally determined. The plate has anouter edge portion extending toward the woofer diaphragm, the platebeing disposed in acoustically satisfactory relation with respect to theconcentric rims of the two speakers. It is possible to have the outeredge of the plate noncircular or non-symmetrical with respect to theaxis of the tweeter. The latter modification may provide differenteffects on the frequency response characteristic along the axis of thespeaker combination or off the axis.

Similarly, while the phasing baffle plate is preferably flat, a dishedplate shape may be used. In such case, having the phasing baffle plateconcave when viewed from the voice coil end of the cone may be moredesirable.

A combination of low and high frequency range speakers embodying theinvention can be marketed like present day speaker combinations, wherethe woofer and tweeter are rigidly secured to each other, the speakersbeing adapted to have the woofer mounting rim bolted to a conventionalbaffle board forming part of the housing or cabinet in which the speakercombination operates. It is understood that the woofer and tweetercombination must be operated in an appropriate mounting for developingits frequency response characteristics. Usually the manufacturer of thespeaker combination prescribes or recommends a desired mounting for thespeaker in a housing or cabinet.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be disclosed inconnection with drawings wherein:

FIG. 1 is a plan view of a speaker combination provided with a phasingbaffle plate embodying the present invention, the broken linessuggesting a mounting for the speaker combination.

FIG. 1A is a plan view of a modified embodiment of the invention.

FIG. 1B is an enlarged detail on line 1B-1B of FIG. 1A.

FIG. 2 is a transverse sectional view along line 22 of FIG. 1, thesection line going around the center bolt for ease of illustration.

FIG. 3 is a frequency response curve of a conentional speakercombination having a 12 inch woofer and a 5 inch tweeter without the newphasing baffle plate, the X axis showing, logarithmically, frequenciesranging from 20 Hz. up to about 20,000 I-Iz., the Y axis showing decibel(dB) values from about to over 100.

FIG. 4 shows a corresponding frequency response characteristic for thesame speaker combination of FIG. 3, the combination in this instancebeing provided with a phasing baffle plate of 45 pound density particleboard having a thickness of substantially three-eights of an inch withconcentric circular inner and outer edges 8-% inches and 4-% inchesdiameters respectively, washers 51 for spacing the phasing baffle platebeing one-sixteenth inch thick, the frequency scale and dB scale beingthe same as FIG. 3.

FIG. 5 shows a frequency response curve of the same speaker combinationas set forth in connection with FIG. 3 wherein voice currents were fedonly to the tweeter, the speaker combination not having the phasingbaffle plate.

FIG. 6 shows the frequency response curve of the tweeter in the samecombination of speakers as before, with the phasing baffle plate, thewoofer being inoperative.

FIG. 7 is a frequency response curve of the same speaker combinationprovided with the phasing baffle plate identified in connection withFIG. 3 with the woofer only operating and the tweeter or highfrequencyspeaker being inoperative.

FIG. 8 is a simple circuit diagram illustrating a network for separatingand feeding high-frequency voice currents to the tweeter.

DESCRIPTION OF A SIMPLE WOOFER AND TWEETER SPEAKER COMBINATION MINUSINVENTIVE FEATURE Permanent magnet 10, generally of toroidal shape, haspole faces 11 and 12. Pole faces 11 and 12 have cemented or otherwisejoined thereto bottom and top pole plates 13 and 14 respectively. Thepole plates are of soft iron or soft steel, with the outer portion ofpermanent magnet projecting beyond plates 13 and 14. The inner edges ofpermanent magnet 10 are laterally and outwardly offset from inner edgeof face 15 of top plate 14. Bottom plate 13 is a solid disc having acountersunk central aperture 17 through which extends bolt 18.Cylindrical pole piece 20 is disposed about the body of bolt 18. Inorder to minimize magnetic reluctance, the opposing surfaces between themagnet and the pole plates as well as pole piece 20 are ground to asmooth finish and the opposing surfaces are cemented together with athin layer of cement.

Resting against the top face of top pole plate 14 is annular flatportion 21 of a steel (usually soft) basket 22 which extends upwardly(as seen in FIG. 2) from flat portion 21 and outwardly to providemounting flange or rim 24. Basket 22 as a rule has windows punched outtherefrom to reduce weight and improve the speaker acoustic properties.

The outer cylindrical surface of pole piece 20 cooperates with innercylindrical pole face 15 of top plate 14 to provide an annular air gapwithin which voice coil 26 operates. As is well known, woofer voice coil26 consists of a number of turns of wire wound over a fabric orsynthetic bobbin of suitable stiffness. During woofer operation, thevoice coil moves axially in response to voice currents, the amplitude ofexcursion being a function of the amplitude of voice currents in thevoice coil. The voice coil operates in a narrow air gap about the outersurface of pole piece 20, the gap defining close spacing between opposedpole surfaces.

The bobbin of the voice coil is securely attached, as by cementing, toannularly corrugated spider 28 whose outer edge 29 is securely attachedto adjacent basket portion 21. The inner edge of spider 28 is alsoattached to small end 30 of woofer cone diaphragm 31. Cone diaphragm 31has its large cone edge 32 attached to or extending into annularlycorrugated cone diaphragm hinge portion 33, the outer edge of which issecurely clamped at rim portion 24 of the basket. Bolts 35 extendthrough openings in rim portion 24 of the basket and cooperate withcardboard ring 36 for tightly clamping the various parts together. Bolts35 are used to secure the rim portion of the speaker basket to a baffleboard of housing for mounting a speaker in a suitable manner.

To protect the woofer voice coil and the air gap in which it operatesfrom dirt and dust, cover 37 of foam rubber or plastic is provided. Foamdust cover 37 is cemented to thecone diaphragm 31 near the small endthereof. Disposed about bolt 18 above the top of pole piece 20 is rigidmouting spacer 39. This spacer is preferably of non-magnetic metal andextends through the central portion of dust cap 37. Boll 18 has itsthreaded end portion 40 screwed into the bottom of a tweeter cone-typespeaker unit to support the same in coaxial relationship with the wooferunit which has just been described. In general, the tweeter speaker unitis a small version of the structure so far described and requires nodetailed description. In this particular tweeter unit, basket 42 doesnot have windows as is true of the woofer basket. However some tweetershave apertures through the basket to change acoustic characteristics.The tweeter basket has mounting rim 43. The voice coil in each of thetwo speakers is connected by wires to an outer terminal board, s'uchwires being led along a path to prevent excessive vibration andcrystallization of the wire metal.

Referring to FIG. 8, a cross-over network for a speaker combination isillustrated. Audio frequency voice currents are fed to wires 61 from asuitable source, usually an amplifier. Wires 61 are connected to voicecoil 26 of a cone diaphragm type of loudspeaker 62, as for example thelow-freqency speaker illustrated in FIG. 2. Branching from wires 61 arewires 64 and 65. Wire 64 is connected through series connectedcapacitors 66 to 67 to one terminal of the voice coil of highfrequencyor tweeter speaker 69. Capacitors 66 and 67 have suitable values forpassing audio frequency currents. Connected between the common junctionof capacitors 66 and 67 is audio frequency inductor 70, the bottomterminal of which is connected to line 65. The voice coil forhigh-frequency tweeter 69 is connected between the free terrninal'ofcapacitor 67 and supply wire 65.

The combination of capacitors 66 and 67 and inductor 70 cooperates topass voice currents having frequencies above about 600 Hz. Generally asharp cutoff for high frequencies is not desirable. However networkssimplicity or complexity may be used. No great precision in establishinga frequency cross-over region is necessary since the large and smallspeakers themselves function to effect frequency separation.

The speaker arrangement so far described is well known and widely used.In the case of a 12 inch speaker, the large cone diameter (usuallymeasured at the outer rim edge of the basket) may be any one of 8inches, 10 inches, 12 inches or I5 inches (these four sizes being quitecommon) and have tweeters which may have large end cone diameters ofabout 3 inches or less for an 8 inch woofer, about 3 or 5 inches for 10and 12 inch woofer or 4 or 6 inches for the tweeter for use with 15 inchwoofers.

As has been previously indicated, for testing a speaker combinationunit, as illustrated in FIG. 2, must be a production unit and must besuitably mounted in a baffle board for testing acoustic response. Asshown by the curve in FIG. 3, a coaxial speaker combination utilizing al2 inch woofer and a 5 inch tweeter tested out with holes at about 1,100Hz. and 2,000 I-Iz., a high resonance peak at about 4,500 Hz., a baddrop at about 7,000 Hz. followed by another bad drop at about 8,000 Hz.and bad drops at about 11,000 and about 13,000 Hz.

To correct these bad drops and objectionable peak at around 4,500 l-Iz.,various expendients had been relied upon. For example the tweeter axialposition can be adjusted without too much trouble to move the tweeterdiaphragm axially along the space within the large diameter cone. Such achange or adjustment however has limited effects and will not smooth outthe usually rough frequency response curve. It had been the practice foran experienced speaker engineer to visually analyze the response curveas FIG. 3 for example) and quess where the trouble is, By playing withthe sizes and nature of various parts making up the speaker and thegauge of metal subject to possible vibration, some changes in thecharacteristic curve may be effected. However such changes are usuallyin the realm of production changes, are expensive, time consuming andmay involve changes in tooling.

Even then, the fundamentally rough frequency response of the two wayspeaker combination still remains. Additionally, makers of equipmenthaving speakers as parts thereof are usually unable to persuade thespeaker manufacturer to improve the frequency response characteristicunless leverage in the form of large orders or general customer demandis present. There are good guality speakers available but the pricesfrequently cannot be justified by the economics of the situation.

Modification of the Speaker Combination in Accordance with the InventionIn order to incorporate the invention in a conventional speakercombination construction as described previously, apertured phasingbaffle plate 50 is provided. This plate is bolted or otherwise securedto rim 43 of tweeter basket 42.

In accordance with the invention, an air path is provided between thetweeter rim and inner edge 54 of phasing baffle plate 50. The size ofthe air path is one of the parameters which must be experimentallydetermined by a skilled speaker engineer to provide a desired frequencyresponse characteristic. A simple means for creating such an air path isillustrated in FIGS. 1 and 2. Spacer washers 51 of suitable material asiron, aluminum, etc. are disposed about bolts 52 between tweeter rim 43and the inner edge portion of plate 50 to secure plate 50 in desiredspaced relation thereto. In the speaker system tested, for whichfrequency response characteristics are given, washers 51 wereone-sixteeth inch thick, for three-sixteenth inch bolts.

No attempt has been made to show parts or air spaces in properproportion in the drawings. The air spaces created by washers 51 mayrange from that created by a thin washer at each bolt 52 to thickwashers, depending upon the circumferential length of all the air spacesdue to washers 51. Large thickness of washers 51 at each bolt 52 may beavoided by removing plate material at the inner edge or adopting adifferent plate support means.

Referring to FIG. 1A, plate 50' has inner edge 54' generated by a foursided window or aperture, the sides being curved outwardly. The apertureis larger than the tweeter rim. Fingers 55 of suitable material (thematerial may be the same as plate material) are securely attached, as bycementing, to one face of plate 50 and extend radially inward to thecone axis overlie tweeter rim 43. Bolts or screws 52' (with or withoutspacer washers) extend through tweeter rim 43 and anchor plate 50 to thetweeter rim. By controlling the shape and dimensions of the window oraperture of phasing baffle plate 50', any desired size for the air patharound inner edge 54' of plate 50' may be provided.

As an example, each finger 55 may be of particle board, three-eighthsinch thick, where it is secured to the bottom face of plate 50', andone-sixteenth inch thick where it extends over tweeter rim 43 and aboutseven-sixteenths inches wide. The length may be about I-iinches. Theouter circular edge of plate 50' may have a diameter of about 8-%inchesand thickness of about one-half inch. The radius of the circle uponwhich the four arcuate sides of the window lay can be substantially 4-/2inches. The window is symmetrical with respect to the circular outeredge, arcuate window sides being similar. The entire plate is usefulwith a 5 inch tweeter having a 12 inch woofer. In this instance, plate50' is bevelled at the outer edge. This is not essential. By reversingfingers 55 prior to cementing or by reversing the entire plate, afterthe fingers are secured, to change plate position, some changes in theresponse curve may be obtained.

In case the tweeter mounting rim is unsuitable for attaching a phasingbaffle plate, it is possible to attach mounting fingers to the plate,such fingers extending outwardly toward the woofer mounting rim and besecured thereto.

It is generally desirable to have the phasing It is generally desirableto have the phasing baffle plate positioned so that it lies within thwoofer cone. If it projects forwardly beyond the woofer, it may be diffcult to dispose a fabric covering for decoration. On the other hand,having the tweeter too deep in the cone is not desirable for reasons ofsound dispersion. In general, conventional testing can be used todetermine the most desirable location.

It has been found that the annular space between the outer edge of plate50 (or 50' and mounting rim 24 of a 12 inch woofer should be at least 1inch and preferably greater so that the low frequency response of theentire speaker combination will not be adversely effected. For an 8 inchwoofer, the annular space may go to one-half inch while a 15 inch woofermay require more than 1 inch space. It has already been pointed out thatplate 50 (or 50 may be made of various materials having the propertiesset forth and that for the most part, depending upon the material, athickness of the order of about one-tenth inch is a minimum. As the testcurve indicate, a plate of 45 pound density particle board having athickness of three-eights of an inch was satisfactory. The thickness ofthe plate is generally not critical.

Referring now to FIG. 4 this curve shows the frequency responsecharacteristic of the speaker combination used in connection with thefrequency response characteristic illustrated in FIG. 3 except that, athreeeights inch thick phasing baffle plate of above identifled particleboard having cconcentric inner and outer circular edges whose diametersrespectively were 8- %and 4-%inches was used, the plate being attachedto the tweeter rim by screws as shown in FIGS. I and 2 of the drawings,spacing washers 51 being one-sixteenth thick. The form of the curveshown in FIG. 4 is generally similar to that of FIG. 3 up to about 1,000Hz. The drop or hole in FIG. 3 at about 1,100 Hz. has disappeared. Thecurve of FIG. 4 has been smoothed out somewhat between 1,000 Hz. andabout 2,000 Hz. The hole or drop at about 2,000 Hz. in FIG. 3 hasdisappeared and the high amplitudes of speaker response in the range offrom about 2,000 to about 6,000 Hz. has been considerably reduced inFIG. 4. The sharp drops or holes in FIG. 3 for 7,000 Hz. and for about11,000 Hz. have disappeared in FIG. 4, a sharp hole or drop in FIG. 4now appearing at about 13,000 or 14,000 Hz. in FIG. 4.

At this high frequency of well over 10,000 Hz. such a drop or hole maynot be objectionable. This drop in FIG. 4 at about 12,000 Hz. can bemoved toward the higher frequency end of the curve if desired by makingsome changes in the phasing baffle plate, such as for example slightlyreducing the diameter of the outer edge. It is also possible to increasethe amplitude of speaker response in the range of'between about 5,000and about 9,000 or 10,000 by suitable operation on the phasing baffleplate. The frequency response curve illustrated in FIG. 4 does showgreat improvement overthe response curve illustrated in FIG. 3. Insofaras quality of speaker response is concerned, the curve illustrated inFIG. 4, as distinguished from the curve illustrated in FIG. 3, reflectsa smoother and substantially better sounding speaker system due to thepresence of the phasing baffle plate. It should be understood that the45 lb. density particle board is one example of a readily availableparticle board. Denser or less dense materials maybe used.

Referring now to the frequency response curve illustrated in FIG. 5,this shows the speaker combination used in connection with obtaining theresponse curve illustrated in FIG. 3, the speaker being a combination of12 inch and 5 inch speakers with no phasing baffle plate and wherein thewoofer is not connected to receive voice currents. It will be noted fromFIG. 5, tha there is little or no response below about 500 Hz. At about1,200 Hz. there is a deep hole while at about something over 2,000 Hz.there is a substantial reduction in amplitude. Beyond that frequency,the response curve illustrated in FIG. 5 is generally similar to thecorresponding frequency range for the curve illustrated in FIG. 3. 1

The frequency response curve illustrated in FIG. 6 shows the operationof the same speaker combination with the tweeter only being operated,the woofer not receiving any voice currents and the tweeter now havingthe phasing baffle plate previously described. It will be noted that theresponse curve from about 800 Hz. to about 12,000 Hz. has been smoothedout substantially.

FIG. 7 shows the frequency response curve of the speaker combinationpreviously identified wherein the phasing baffle plate is provided andthe woofer only operating. the tweeter being inoperative. The frequencyresponseup to about 800 Hz. is generally similar to the frequencyresponse curve of FIG. '4 wherein both speakers are operating with FIG.7, the response characteristic falls off quite rapidly. It is evidentthroughout the characteristic curve illustrated in FIG. 4 that thetweeter response, when operating in conjunction with the woofer, in thecombination having the phasing baffle plate, shows that the contributionof the tweeter in this combination begins to be significant from about600 Hz. and up and that the tweeter response or contribution to thespeaker response begins to be the major factor in frequencies beyondabout 2,000 Hz.

Adapting The Phasing Baffle Plate To aSpeaker Combination For ObtainingThe Benefits of the Invention To adapt a phasing baffle plate to acoaxial speaker combination and obtain the benefits of the invention, itis necessary first to obtain with a conventional speaker combinationhaving large and small'production speakers disposed in fixed relation toeach other as illustrated in FIG. 2 of the drawings. Thereafter aphasing baffle plate must be adapted to the speaker. It is understoodthat the speaker combination minus phasing baffle plate will be testedto show frequency response characteristics over the entire frequencyrange to be covered by the speaker combination. The test should be bothalong and off the cone axis. Then a suitable material for the phasingbaffle plate, such as particle board, must be selected. Assuming thatconcentric outer and inner circular edges are to be used, select theplate material having the proper thickness, (for 45 lb. density particleboard, a thickness of three-eighths inch will serve as a starter) cutthe window through the plate. This may be a circular opening or windowwhose diameter is large enough to permit mounting the phasing baffleplate upon the tweeter rim as shown in FIGS. l and 2. Then arbitrarilytrim plate material so that the outer edge will have a diameter 2 inchesgreater than that of the window. The entire speaker combination together with the experimental phasing baffle plate must be suitablymounted in a large baffle board either in a housing or cabinet or if thebaftle board is large enough, the entire construction can be tested inopen air. A conventional frequency response test is now run and theresponse curve is plotted. This response curve should be compared to theresponse curve of the same speaker combination with similar baffle boardbut without the phasing bafile plate. Different thicknesses of washers51 should be tried. Thereafter other baffle plates having increasingdiameters in steps of onefourth inch for the outer edge should besubstituted in the speaker combination. Beyond a certain point in thevarious phasing baffle plate sizes and spacings there will usually be anoptimum response curve, the curve beginning to deteriorate beyond acertain outer edge diameter of the phasing baffle plate and/or spacerwasher thickness. At or close to the best dimension for the phasingbaffle plate, some small variations in diameter of both inner and outeredges of phasing baffle plate may be tried to determine the mostdesirable frequency characteristic curve. Due to the number ofvariables, it will generally be desirable to vary only one or twoparameters at a time while holding others constant. It is possible toobtain satisfactory frequency response characteristics for a speakercombination for a number of parameter values. Some parameters havegreater response effects than others. In general, the air space attweeter rim-43 and space between the plate edge and woofer cone areimportant. Trying different tweeter locations along the cone axis mayalso be investigated. Once a desirable phasing baffle plate is obtained,no changes in the speakers should be made to avoid making changes in theplate.

Once the parameters of the phasing baffle plate are determinedexperimentally, production of phasing baffie plates determined by theexperiments can proceed. It will be found that a substantial freeannular region between the outer edge of the phasing baffle plate andthe nearest portion of the woofer cone will be indicated, such annularspace as a rule having at least-a width radially of more than one-halfinch between the edge of the phasing baffle plate and adjacent woofercone. Inasmuch as phasing baffle plates are easy to make and the platematerial and their dimensons are easily changed, testing of variousphasing baffle plates may be accomplished economically and quickly.Frequency response tests for speakers are well known.

In testing a speaker combination embodying the present invention,variations in the shape of the outer edge of the phasing and baffleplate, such as indentations, will produce different effects on thefrequency response characteristic. In making such changes in the phasingbaffle plate it may be desirable to test the speaker response not onlyalong the cone axis of the speakers but laterally away from the axisfrom a region about 4 or more feet along the axis from the speaker.

The modified phasing baffle plate shown in FIG. 1A may be used withequal convenience. Desirable plate shapes, dimensions, and air spacesmay be determined by tests. A skilled speaker engineer will be able todetermine in which direction to change plate dimensions, plate density,air spacing, etc. to improve speaker response.

What is claimed is:

l. A self-contained loudspeaker combination comprising a cone-diaphragmtype woofer and a conediaphragm type tweeter, each cone-diaphragmcarrying a voice coil operating in an annular air gap of a magneticcircuit and having a stationary mounting rim for supporting an annularhinge portion at the large cone end, the woofer being for low-frequencyoperation and having its large end cone diameter of at least about 8inches, the tweeter being for higher frequency operation and having itslarge end cone diameter substantially less, means for supporting thetweeter in coaxial nested relation within the volume defined by thewoofer cone, and a pushing self-supporting flat baffle plate ofmaterial, whose thickness is between about one-tenth and aboutthree-fourths inch, said plate having a curved outer edge and a windowtherethrough to provide an inner edge, means for supporting said platein symmetrical relation to the coincident diaphragm axes andperpendicular thereto, said plate being small enough to be supportedwithin the large diaphragm end portion with its outer edge stoppingshort of the large cone rim portion by between about one-half inch tothe order of about 1 inch, depending upon the woofer speaker size, theplate inner edge being dimensioned and located with reference to thetweeter diaphragm mounting rim so that an acoustically suitableperipherally continuous extended air gap is provided, said platematerial having suitable acoustic properties and the plate and spacingdimensions, shape and axial location being so selected that said speakercombination, with phasing baffle plate, when mounted for normal speakeroperation with a conventional baffleboard will have a desired uniformfrequency response over the desired audio-frequency range and besubstantially free of resonance peaks or holes having abnormalamplitudes in such range, even though the same speaker combination minusthe phasing baffle plate, when tested, will have at least one resonanceor anti-resonance peak of abnormally large amplitude within theaudio-frequency range, said phasing baffle plate smoothing out thefrequency response characteristic and improving speaker performance, theaddition and mounting of said phasing baffle plate permittingengineering efforts to be directed thereon to improve frequency responsewhereby tooling changes for manufacturing the speaker combination propermay be substantially eliminated.

2. The construction according to claim 1 wherein the woofer conediameter ranges from about 8 inches to about 15 inches, said phasingbaffle plate having its outer edge circular with the center being on thecone axis.

3. The construction according to claim 1 wherein the plate is ofparticle board.

4. The construction according to claim 1 wherein the plate is ofstyrofoam having a smooth, moulded outer surface.

1. A self-contained loudspeaker combination comprising a conediaphragmtype woofer and a cone-diaphragm type tweeter, each cone-diaphragmcarrying a voice coil operating in an annular air gap of a magneticcircuit and having a stationary mounting rim for supporting an annularhinge portion at the large cone end, the woofer being for low-frequencyoperation and having its large end cone diameter of at least about 8inches, the tweeter being for higher frequency operation and having itslarge end cone diameter substantially less, means for supporting thetweeter in coaxial nested relation within the volume defined by thewoofer cone, and a pushing self-supporting flat baffle plate ofmaterial, whose thickness is between about one-tenth and aboutthree-fourths inch, said plate having a curved outer edge and a windowtherethrough to provide an inner edge, means for supporting said platein symmetrical relation to the coincident diaphragm axes andperpendicular thereto, said plate being small enough to be supportedwithin the large diaphragm end portion with its outer edge stoppingshort of the large cone rim portion by between about one-half inch tothe order of about 1 inch, depending upon the woofer speaker size, theplate inner edge being dimensioned and located with reference to thetweeter diaphragm mounting rim so that an acoustically suitableperipherally continuous extended air gap is provided, said platematerial having suitable acoustic properties and the plate and spacingdimensions, shape and axial location being so selected that said speakercombination, with phasing baffle plate, when mounted for normal speakeroperation with a conventional baffleboard will have a desired uniformfrequency response over the desired audio-frequency range and besubstantially free of resonance peaks or holes having abnormalamplitudes in such range, even though the same speaker combination minusthe phasing baffle plate, when tested, will have at least one resonanceor anti-resonance peak of abnormally large amplitude within theaudio-frequency range, said phasing baffle plate smoothing out thefrequency response characteristic and improving speaker performance, theaddition and mounting of said phasing baffle plate permittingengineering efforts to be directed thereon to improve frequency responsewhereby tooling changes for manufacturing the speaker combination propermay be substantially eliminated.
 2. The construction according to claim1 wherein the woofer cone diameter ranges from about 8 inches to about15 inches, said phasing baffle plate having its outer edge circular withthe center being on the cone axis.
 3. The construction according toclaim 1 wherein the plate is of particle board.
 4. The constructionaccording to claim 1 wherein the plate is of styrofoam having a smooth,moulded outer surface.